Conventional mechanically driven valve trains operate the intake and exhaust valves based on the position and profile of lobes on a camshaft. The engine crankshaft is connected to the pistons by connecting rods and to the camshaft by a belt or chain. Therefore, the intake and exhaust valve opening and closing events are based on the crankshaft position. This relationship between the crankshaft position, piston position, and valve opening and closing events determines the stroke of a given cylinder, e.g., for a four stroke engine the intake, compression, power, and exhaust strokes. As a result, engine starting and the first cylinder to fire are determined in part by the camshaft/crankshaft timing relationship and the engine stopping position.
On the other hand, electromechanically driven valve-trains do not have the physical/constraints that tie the camshaft and crankshaft together, i.e., there may not be belts or chains linking the camshaft and crankshaft, at least for some valves. Furthermore, full or partial electromechanical valve-trains may not require a camshaft. Consequently, the physical constraints linking the camshaft and crankshafts are broken. As a result, additional flexibility to control valve timing is possible when electromechanical valves are used in an internal combustion engine.
One method to control electromechanical valve operation during an engine operation is described in U.S. Pat. No. 5,765,514. This method provides for an injection sequence for the cylinders that is initialized when a first crankshaft pulse is generated after generation of a first signal pulse representing crankshaft rotation through 720 degrees. The injection sequence and crankshaft position sequence correspond to the position of each cylinder, whereby the opening/closing timing of each intake valve and exhaust valve can be controlled. The cylinders are set to the exhaust stroke, suction stroke, compression stroke, and explosion stroke, respectively.
Once the above-mentioned method has set the stroke of each cylinder, valve sequencing follows the cylinder assigned stroke pattern until the engine stops and the engine is restarted. In other words, with the exception of closing valves before valve synchronization during starting, the method schedules engine valve events consistent with a conventional camshaft constrained engine. After valve synchronization, valve operation begins by initiating an exhaust stroke in respective cylinders. Therefore, engine emissions and starting are expected to be similar to conventional cam based engines since the approach focuses on operating cylinder valves based on four-stroke engine operation and crankshaft position.
The inventors herein have recognized several disadvantages of this approach related to the potential for fuel to remain in the intake manifold after the engine is shut-down. Specifically, fuel remaining in the intake manifold from the last engine operating cycle before shut-down may negatively influence subsequent starting torque and emissions. Further, fuel remaining in the intake manifold may also contribute to increasing evaporative emissions.